Proportional throttle valves are generally used in fluid systems for controlling volumetric flows. In this connection, directly controlled valves in which a proportional magnet system acts directly on the control piston, as in a switching directional control valve, have proven less suited in those cases in which large volumetric flows, especially at high differential pressures, must be controlled. Flow forces may arise that superimpose themselves on the actuating forces of the magnet system to lead to malfunctions.
To remedy this problem, state of the art valves of the initially indicated type for these applications, i.e., those proportional throttle valves having hydraulic piloting, are used. When using these conventional valves, problems arise with respect to operating behavior. Although mechanical decoupling of the valve piston from the magnet system helps to reduce instabilities, because the valve piston is moved solely by fluid pressure, the precision and reliability of operation depend largely on the state of the flow baffle device of the pilot system. In use in a fluid system in which the fluid is not free of fouling, clogging of the baffle bore of the baffle device would lead to uncontrolled opening of the valve presenting a safety hazard. To prevent this danger, the baffle bore is typically made relatively large, with a diameter of more than 0.5 mm. Since the diameter of the pilot valve seat must be chosen to be larger than the diameter of the baffle bore to perform the valve function, several problems arise. One major disadvantage is that large diameters of the baffle bore and pilot valve seat lead to a large pilot volumetric flow. For correspondingly large pressure differences, this volumetric flow can be several liters/min. Precision control of the valve is not possible in this volumetric flow range.
The differential pressure prevailing on the pilot valve seat and on the valve cone of the actuating member, which cone interacts with the valve seat, produces a resulting force that seeks to keep the valve cone in the closed position. In particular, at high differential pressures a relatively large dead flow is thus necessary to raise the actuating member off the pilot valve seat. After the pilot valve seat is cleared, a rapid pressure drop occurs on the back of the piston to greatly reduce the force acting on the actuating element. In the presence of a large magnetic force remaining unchanged, an excess of force of the magnet system arises. It acts against the force of a spring arrangement conventionally pressurizing the actuating element against the pilot valve seat, as a result of which an opening motion of the valve piston takes place with undesired suddenness.